feat: add support for setting custom udp listen port on the xtcp mode

[zjay1987]

Enhancing xtcp Connectivity with UDP Port Forwarding

If the home router’s NAT traversal (hole punching) doesn’t work well—such as when it operates in Symmetric NAT mode only—an explicit UDP port forwarding rule can be configured on the router to make the NAT on the configured port deterministic. For example:

Router Configuration

UDP 9999 → {Your-LAN-IP}:9999

frpc Configuration

[[visitors]]
name = "..."
type = "xtcp"
serverName = "..."
secretKey = "..."
bindAddr = "..."
bindPort = "..."
udpListen = "0.0.0.0:9999"

With this setup, frpc will use UDP port 9999 for communication with the STUN server instead of selecting a random port. Since port 9999 on the router is already configured to forwarded all traffics to {Your-LAN-IP}:9999, if there's no other Symmetric NAT above the router, a successful xtcp connection will be established.